Quick answer
A fixed-rate mortgage keeps the same interest rate for the loan term, so scheduled principal and interest remain predictable. An adjustable-rate mortgage, or ARM, normally starts with a fixed introductory period and then adjusts under the note’s index, margin and caps. Compare written Loan Estimates and test the highest permitted payment—not only the introductory rate.
The difference that matters most
A fixed-rate loan transfers interest-rate uncertainty to the lender: the borrower’s note rate does not change. An ARM transfers part of that uncertainty to the borrower after an initial fixed period. Property taxes, homeowners insurance and other escrow items can still change under either loan, so “fixed” does not always mean the total amount leaving the household account will never move.
An ARM may offer a lower initial rate than a comparable fixed-rate option, but the tradeoff is a less certain future payment. The Consumer Financial Protection Bureau warns borrowers not to assume they will be able to sell or refinance before the rate changes. A stronger comparison asks what happens if the borrower keeps the loan.
Fixed rate vs. ARM at a glance
| Question | Fixed-rate mortgage | Adjustable-rate mortgage |
|---|---|---|
| When can the note rate change? | It generally does not change during the term | After the initial period and at stated intervals |
| What drives the future rate? | The rate agreed at closing | A stated index plus margin, subject to caps |
| Is principal and interest predictable? | Yes, for a standard fully amortizing loan | Only during the initial fixed period; later payments can change |
| Main planning strength | Long-term payment certainty | Potentially lower initial cost |
| Main planning risk | Paying more initially if comparable ARM pricing is lower | Future payment increases and uncertain total interest |
Read the ARM terms as a system
An ARM label such as 5/1 or 5/6 describes the initial fixed period and the later adjustment frequency, but the label is only a starting point. The note and program disclosure identify the index, margin, adjustment dates and caps. The fully indexed rate is generally the index plus the margin, limited by the contract’s caps.
- Initial adjustment cap: limits the first change after the introductory period.
- Subsequent cap: limits later periodic changes.
- Lifetime cap: limits how far the rate can rise over the loan’s life.
- Floor: may limit how far the rate can fall.
- Index and margin: determine how an uncapped adjustment would be calculated.
Ask the lender to point to each term in the written disclosure and show the payment at the maximum first adjustment and lifetime cap. A verbal estimate is not a substitute for the note and Loan Estimate.
Build a payment stress test
Start with the loan amount, introductory principal-and-interest payment and the first possible adjustment date. Add taxes, insurance, HOA dues and a maintenance reserve to see the full housing budget. Then repeat the budget using the largest payment allowed at the first adjustment and the maximum payment shown in the disclosure.
- Confirm the exact introductory period and first adjustment month.
- Record the initial, periodic and lifetime caps.
- Ask for the maximum principal-and-interest payment in dollars.
- Test that amount against today’s reliable income, not hoped-for future income.
- Keep emergency reserves outside the down payment and closing-cost budget.
Use the total monthly ownership-cost guide to include expenses beyond the mortgage and the amortization guide to understand how principal and interest develop over time.
Compare lender offers fairly
Request fixed-rate and ARM Loan Estimates with the same loan amount, down payment, term and lock period on the same day. Compare the interest rate, APR, points, lender credits, loan costs, cash to close and projected payments. An offer with lower cash to close may carry a higher rate, while an ARM with a low initial payment may create more exposure later.
If you are considering points or credits, use the points-versus-credits guide. Do not mix a temporary buydown with an ARM comparison: a 2-1 buydown changes early payments without making the note rate adjustable.
Questions to take to the lender
- What index does this ARM use, where is it published and what is today’s value?
- What margin is added after the introductory period?
- What are the initial, subsequent and lifetime rate caps?
- Can the rate decrease, and is there a floor?
- What is the highest possible payment at the first change and over the loan’s life?
- Do the fixed and adjustable quotes use identical assumptions and lock periods?
Frequently asked questions
Is an ARM always cheaper than a fixed-rate mortgage?
No. An ARM may start with a lower rate, but pricing varies and later adjustments can change the payment and total interest.
Can escrow make a fixed-rate payment rise?
Yes. The note rate and scheduled principal-and-interest payment can remain fixed while taxes or insurance collected through escrow change.
Should I choose an ARM if I expect to move soon?
A shorter ownership plan can be relevant, but a sale date and price are never guaranteed. Test the loan as though you may need to keep it beyond the introductory period.
Sources
- CFPB: Fixed-rate and adjustable-rate mortgage differences
- CFPB: Adjustable-Rate Mortgages
- CFPB: Shopping for a Mortgage
Sources were checked on July 28, 2026. Policy terms, lending practices, product availability and state or local requirements can change.